Organic Chemistry · Organic Compounds: Alkanes and Their Stereochemistry
Alkyl Groups
On this page 9 sections
In 30 seconds
An Alkyl group Alkane fragment with one H removed and one free valence (R-) Full entry → is what you get when you remove one hydrogen atom from an alkane: a fragment with one unsatisfied ("free") valence that can attach to the rest of a molecule. The general symbol is R-, and the general formula is:
CnH2n+1
Removing an H from methane (CH4) gives a methyl group, CH3-; from ethane, an ethyl group, CH3CH2-; from propane, two different groups are possible depending on which H is removed — an n-propyl group (CH3CH2CH2-) or an isopropyl group ((CH3)2CH-). Propane already shows the central idea of this topic: the position of the Free valence The unsatisfied bonding position left where H was removed Full entry → determines which alkyl group you have. Butane gives four distinct groups — n-butyl, sec-butyl, isobutyl, and tert-butyl — whose names you must be able to draw from memory. Because alkyl groups appear as substituents in nearly every organic molecule, recognizing them instantly is an essential skill for naming compounds (next topic) and for understanding how structure affects reactivity throughout the course.
Why this matters
- Naming: Alkyl groups are the most common substituents in IUPAC names (2-methylbutane, 4-ethyloctane, tert-butyl…). You cannot name compounds without recognizing them.
- Common names: Drug names and everyday chemical names lean on alkyl-group vocabulary — ibuprofen is an isobutyl-substituted molecule, and "tert-butyl" appears constantly in laboratory reagents.
- Reactivity logic: Alkyl groups donate electron density through σ bonds (inductive effect) and stabilize carbocations and radicals — a theme that explains Markovnikov's rule and many reaction rates in later chapters.
- Structure drawing: The shorthand R lets chemists write "a reaction that works on any alkyl group" without drawing every possible chain; understanding R unlocks every general reaction you will study.
- Exams: Drawing all isomers of a given alkyl group, converting between group name and structure, and counting hydrogens in CnH2n+1 fragments are standard questions.
The college version
Core Concepts
Defining an alkyl group
An alkyl group is an alkane minus one hydrogen atom. It is not a stable, isolable molecule on its own — it always carries a bond to something else (another carbon, a halogen, an oxygen…). Think of it as a molecular "handle" with one open hand. The open hand is the free valence, and it can be located at the end of a chain (giving a primary group like n-propyl) or in the middle (giving a secondary group like isopropyl or sec-butyl).
The general formula CnH2n+1
Removing one H from an alkane CnH2n+2 leaves 2n + 1 hydrogens. So a methyl group is CH3 (1 C, 3 H), ethyl is C2H5, propyl groups are C3H7, and all butyl groups are C4H9. All isomers of a given alkyl group share the same formula — what differs is where the free valence sits.
The two propyl groups
Propane, CH3CH2CH3, has two kinds of hydrogens:
- Remove a terminal H → n-propyl (normal propyl), CH3CH2CH2-, free valence on a terminal carbon (primary).
- Remove the middle H → isopropyl, (CH3)2CH-, free valence on the central carbon (secondary).
The "n-" (normal) prefix means a straight chain with the free valence at the end; "iso-" means a branched group whose free valence sits at the end of a chain ending in a (CH3)2CH- branch.
The four butyl groups
Butane and isobutane combine to give four distinct C4H9 groups:
- n-Butyl: CH3CH2CH2CH2- — straight chain, free valence at the end (primary).
- sec-Butyl: CH3CH2CH(CH3)- — straight chain, free valence on the second carbon (secondary).
- Isobutyl: (CH3)2CHCH2- — branched, free valence at the end (primary).
- tert-Butyl: (CH3)3C- — three methyls on one carbon, free valence on the central carbon (tertiary).
The prefixes n-, sec-, iso-, and tert- are common-name conventions. In IUPAC naming (next topic), the same groups are named systematically — for example, isopropyl becomes "1-methylethyl" and tert-butyl becomes "1,1-dimethylethyl."
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| n-Propyl (CH3CH2CH2-) | Isopropyl ((CH3)2CH-) | Free valence at the chain end (primary) vs. on the middle carbon (secondary); both are C3H7 |
| sec-Butyl | Isobutyl | Straight chain with free valence on C2 vs. branched chain with free valence at the end |
| tert-Butyl | n-Butyl | Three methyls on a central carbon (tertiary, very bulky) vs. straight chain with free valence at the end |
| Alkyl group | Alkane | An alkyl group has one fewer H and an open valence; it is a fragment, not a stable standalone molecule |
| iso- prefix | Any branched group | "iso-" specifically means an end-of-chain (CH3)2CH- arrangement — not every branch is "iso" |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a LEGO chain of carbon bricks with hydrogen studs. An alkyl group is the chain with one stud popped off, leaving an empty connector — that's the free valence, ready to snap onto another piece. If you pop a stud off the end of a three-brick chain you get an "n-propyl" group; if you pop the stud off the middle brick instead, you get an "isopropyl" group — same bricks, different empty spot, different name. The empty spot's location is everything.
Worked example
Example 1: Counting hydrogens in an alkyl group
How many hydrogens are in a tert-butyl group?
Write the general formula first:
CnH2n+1
A tert-butyl group has 4 carbons, so substitute n = 4:
H = 2(4) + 1 = 9
Check by drawing: (CH3)3C- has three methyls (3 × 3 = 9 H) on a central carbon that carries no H of its own — 9 H total, matching C4H9.
Example 2: Drawing every butyl group from a name
Draw isobutyl and sec-butyl, and state where each free valence sits.
Isobutyl is (CH3)2CHCH2-: a three-carbon chain with a methyl branch on carbon 2 and the free valence on the terminal carbon — a primary group. sec-Butyl is CH3CH2CH(CH3)-: an unbranched four-carbon chain with the free valence on carbon 2 — a secondary group.
Answer: isobutyl = primary, free valence at the end of a branched chain; sec-butyl = secondary, free valence in the middle of a straight chain. They are different groups even though both are C4H9.
Example 3: Identifying alkyl groups in a molecule
Identify the alkyl substituents in 2,2,4-trimethylpentane (the gasoline octane-rating standard, "isooctane").
Structure: (CH3)3CCH2CH(CH3)CH3. The main chain is five carbons (pentane). On carbon 2 there are two methyl groups, and on carbon 4 there is one methyl.
Answer: the substituents are three methyl groups — two on carbon 2 and one on carbon 4. No ethyl or propyl groups appear; the name "trimethyl" already tells you all three substituents are methyl.
Key takeaways
- Alkyl group = alkane − one H; formula CnH2n+1; symbol R-.
- Methyl CH3-, ethyl CH3CH2-, n-propyl CH3CH2CH2-, isopropyl (CH3)2CH-.
- Four butyl groups: n-butyl, sec-butyl, isobutyl, tert-butyl — know how to draw each.
- The free valence position defines the group: end of chain = primary (n-), middle carbon = secondary (sec-/iso-), central carbon with three methyls = tertiary (tert-).
- Propyl and butyl isomers share formulas (C3H7, C4H9) but differ in connectivity.
- Common prefixes (iso-, sec-, tert-, n-) are shorthand; IUPAC systematic names describe the same groups explicitly.
- Count hydrogens with 2n+1: C4H9 has 9 H regardless of which butyl isomer it is.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Write the general formula for an alkyl group with n carbons, and use it to find the formula of an ethyl group.
Show answer
CnH2n+1; for ethyl, n = 2, so C2H5 (CH3CH2-).
Draw n-propyl and isopropyl groups and state where each free valence is located.
Show answer
n-Propyl: CH3CH2CH2-, free valence on the terminal carbon (primary). Isopropyl: (CH3)2CH-, free valence on the middle carbon (secondary).
How many distinct C4H9 alkyl groups exist? Name them.
Show answer
Four: n-butyl, sec-butyl, isobutyl, tert-butyl.
Which alkyl group is tertiary? Draw it.
Show answer
tert-Butyl, (CH3)3C- — the central carbon is bonded to three other carbons.
Are n-butyl and sec-butyl constitutional isomers? Explain.
Show answer
Yes — same formula C4H9 but different connectivity: n-butyl is a straight chain with the free valence at the end; sec-butyl has it on carbon 2.
A substituent has the structure (CH3)2CHCH2-. What is its common name?
Show answer
Isobutyl — a branched chain with the free valence at the end.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Alkyl group
- Alkane fragment with one H removed and one free valence (R-)
- Free valence
- The unsatisfied bonding position left where H was removed
- Primary (1°) group
- Free valence on a terminal carbon of an unbranched chain
- Secondary (2°) group
- Free valence on a carbon bonded to two other carbons
- Tertiary (3°) group
- Free valence on a carbon bonded to three other carbons
- n- prefix
- "Normal": straight chain, free valence at the end
- iso- prefix
- Branched group ending in (CH3)2CH-
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.